Accmmt of Dr. Parr's Theory of Light and Heat, 



VI. 



On Dr. Pan's Theory of Light and Heat. By a CorreJ^ondent. 



To Mr. NICHOLSON. 



Ac 



SIR, 



XCIDENT prevented my feeing, for fome time, your Journal, publiflied in December 

 laft; and I now take the liberty of making a few obfervations on fome remarks of your corref- 

 pondent, refpe£ting Dr. Parr's theory of light and heat*. He obferves, " with refpeft to Dr. 

 Parr's theory, that light and fire repel each other, when difengaged from matter, it appears 

 to me too fanciful to be folid, and too inconfiftent to be fupported," &c. — I do not quote the 

 whole paffage, becaufe I mean not to commence a controverfy upon tlie fubje£l:, as from the 

 obfervation juft tranfcribed, as well as from what follows, it is evident that your correfpondent 

 has feen only the quotation in Dr. Pearfon's paper ; and as the eflay was publiflied before 

 the appearance of your Journal, I requeft only room for a Ihort analyfis of the theory, that 

 it may not be, as I fufpedt it has been in the paflage before me, mifunderftood. I {hall take 

 up a little fpace only in your valuable publication, but you will allow me to add, that M. 

 Gadolin has fince publifhed an opinion not very different ; and that, when Count Rumford 

 fpeaks of changes effefted, not " by any chemical combination of the matter of light with 

 fuch bodies (as are expofed to it), but merely by the heat which is generated or excited by the- 

 light which is abforbed by them" he in reality ufes the fame language. 



Dr. Parr does not merely affert an opinion, but fupports his hypothefis by mjmerous and 

 well-connedted fafts. Light he confiders, with the chemical philofopher of the prefent day, 

 as a fubftance capable of combining with the various bodies it meets, producing by that 

 means many chemical changes. Among thefe, he traces with peculiar care its influence 

 upon vegetation ; and from the decompofition of water, afforded in the leaves of vegetables, 

 feemingly by its power, he explains the great variety of gafes expired by plants in different 

 fituations. 



As light thus feparates oxygen, and as the latter probably contains heat in a lefs com- 

 pounded ftate than any other fubftance, he fteps over the next difficulty, and fuppofes that 

 it feparates heat. This is the only ftep unfupported by immediate fa£l:s ; but if affumed for 

 a moment as an hypothefis, the concurrence of numerous other fa£ts will raife it to a higher 

 rank. By what means light and heat are brought together in union, when they are ingre- 

 dients in inflammable bodies, he pretends not to explain ; but if they are really different ele- 

 ments, as is now generally believed, our not knowing the bond of union, is no proof againft 

 its exiftence. In the examination of the various explofive fubftances. Dr. Parr traces iix 

 each the fource of its light and heat, and (hows by what means the balance fubfifting be- 

 tween thefe aftive ingredients is deftroyed, fometimes by the addition of the one, fometimes 

 by that of the other principle. There are various explofive fubftances which will not ex- 



* ECTay upon Light, in a mifctllaneous volume, publilhcd by » fccicty at Exeter. 



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